Photodetector responsive to light intensity in different spectral bands



Nov. 11, 1969 N. 5. DILLMAN 3,478,214

PHOTODETECTOR RESPONSIVE TO LIGHT INTENSITY IN DIFFERENT SPECTRAL BANDS Filed Feb. 16, 1966 VOLTAGE DETECTOR INCIDENT UGHT INVENTOR. NORMAN G. DILLMAN E'ZQwaAQ AGENT FIG.2

United States Patent US. Cl. 250-211 9 Claims ABSTRACT OF THE DISCLOSURE" A photodetector device is provided comprising a plurality of successive layers of semiconductor material having a band gap energy greater than the preceding layer. A plurality of electrical ohmic contacts corresponding in number to the plurality of semiconductor layers are connected one to. each layer so as to provide for the pickoff of signals.

This invention relates to a photodetectordevice and more particularly it relates to a photodetector which provides an output signal proportional to light intensity in different spectral bands.

An object of the present invention is to provide an improved photodetector device.

Another object of the present invention is to provide a photodetector device which will provide output signals indicative of the spectral content of the light impinging on it.

Another object of the present invention is to provide a photodetector device which will provide output signals that are proportional to light intensity in different spectral bands.

These and other objects of the invention will become more apparent when taken with the following description and in conjunction with the drawings in which:

FIG. 1 is a top view of the preferred embodiment of the invention; and

FIG. 2 is a cross-sectional view of the embodiment of FIG. 1 taken along the lines 22, looking in the direction of the arrows.

In accordance with the present invention and the attainment of the preceding objects a photodetector device is provided comprising a plurality of successive layers of semiconductor material each of said successive layers of semiconductor material having a band gap energy greater than the preceding layer. A plurality of electrical ohmic contacts corresponding in number to the plurality of semiconductor layers are connected one to each layer so as to provide for the pickoff of signals.

Referring to FIGS. 1 and 2, the substrate 10 of conductive material provides the base on which a thin disclike layer of semiconductor material 13 is grown or deposited. A thin disc-like layer of semiconductor material 12 is grown or deposited onto layer 13 so as to form a heterojunction with layer 13. The band gap energy of the material of layer 12 is greater than that of the material of layer 13.

A thin disc-like layer of semiconductor material 11 is grown or deposited onto layer 12 forming a second heterojunction with layer 12; the diameter of the disc of layer 11 is less than the diameter of discs 12 and 13. The band gap energy of the material used in layer 11 is greater than that of the material of layer 12. An ohmic contact is attached to layer 12 and a similar ohmic contact 14 is attached to layer 11. The ohmic contacts 14 and 15 are ring-like in shape.

More generally; in order to obtain N heterojunctions, N +1 different semiconductor materials are grown or deposited in thin layers on the conducting substrate 10 which serves as an ohmic contact and also provides support for the successive layers of semiconductor material. To each layer an ohmic contact is attached so that the voltage generated by each heterojunction may be detected. If the semiconductor material used for the layers were of the photoresistive type rather than of photoelectric type the contacts could be used to detect the change in resistance of the heterojunctions.

Now, letting the subscript i denote a certain semiconductor material starting with 1 at the surface or furthest layer from the substrate and continuing consecutively to layer N at the substrate. The spectral band detected at the i, i+l junction is given by:

if E E i.e., the band gap energy is always closer to the surface, where wherein A is the Wavelength E is the band gap energy h is Plancks constant, and c is the speed of light The absorption edge of the surface material is at a shorter wavelength than the remaining layers and it will be transparent to wavelengths longer than the absorption edge wavelength to a first approximation. The same argument holds for each layer. The light absorbed at the i-th layer is detected as a photovoltage across the (il, i) junction.

Referring to the photodetector of FIG. 2 which has two junctions. If E i E E then the voltage V is proportional to the incident light intensity between the wavelengths =hc/E, and =hc/E The spectrum for V is from A =hc/E to k =hc/E By dividing the spectrum into two bands, such a photodetector could be used to roughly determine star color. Another photodetector with three heterojunctions that trisected the visible spectrum (that is, responded to those colors) could be used to determine visible color.

Semiconductor material that may be used for layers 11, 12 and 13 are listed below along with their respective band gap energies.

In the particular embodiment shown, the layers of semiconductor material are disc-like in construction but it is to be understood that such shape is not to be construed as being restrictive. Other shapes may be used without departing from the scope of the invention. The ohmic contacts are shown as ring-like in shape but again it would be obvious to use segmented contacts or point type contacts to perform the same function.

It is therefore intended that the invention only be limited in spirit and scope by the appended claims.

What is claimed is: 1. A photosensitive device comprising: a plurality of successive layers of partially transparent, semiconductor material each successive layer having a band gap energy greater than the preceding layer;

means for projecting light on the device in the direction of decreasing band gap energy;

means for determining the light intensity impinging upon the photosensitive device in different spectral bands, said means comprising means for determining the voltages respectively across one or more of said layers.

2. The device of claim 1 wherein said voltage is detected across adjacent layers.

3. The device of claim 1 wherein said plurality of layers form a plurality of heterojunctions.

4. The device of claim 1 wherein said layers of semiconductor material are grown one upon the other so as to form a photodetector device which is a single crystal.

5. The device of claim 1 wherein one of said layers is, silicon, another is gallium phosphide, and another is boron zinc sulfide.

6. The device of claim 1 wherein the material of said successive layers are designated by the subscript i and each layer is designated successively 1 through N such that the spectral band detected between the i, i+l layers is determined by the formula;

wherein where A is the wavelength E is the band gap energy h is Plancks constant, and c is the speed of light.

7. A photodetector device comprising: a conductive layer forming a first electrical contact; a first layer of partially transparent semiconductor material deposited onto said conductive layer; a second layer of partially transparent semiconductor material having a hand gap energy greater than the band gap energy of said first layer of semiconductor 3 material deposited onto said first layer;

a second electrical contact in ohmic connection with said second layer;

a third layer of partially transparent semiconductor -material having a band gap energy greater than the band gap energy of said second layer of semiconductor material deposited onto said second layer;

means for projecting light on the device in the direction of decreasing band gap energy;

a third electrical contact in ohmic connection with said third layer; and

means for determining the light intensity impinging upon the photodetector device in different spectral bands, said means comprising means for determining the voltages respectively across one or more of said layers.

8. Aphotodetector comprising: I

a plurality of successive layers of partially transparent semiconductor material forming a plurality of successive heterojunctions, said successive heterojunctions being responsive to successive absorption wavelengths of said layers;

means for projecting light on the photodetector in the direction of increasing absorpiton wavelengths;

means for determining the light intensity impinging upon the photodetector in dilferent spectral bands, said means comprising means for determining the voltages respectively across one or more of said layers.

9 The combination recited in claim 8 wherein said References Cited UNITED STATES PATENTS 8/1960 Jackson 250211 X 6/ 1967 Weinstein 317-235 X WALTER STOLWEIN, Primary Examiner US. Cl. X.R. 

